I have been driving a FiST since 2014. Can't compare it to the BRZ/FRS/GT86 triplets, but I can tell you about daily driving it.
I drive it through the winter on Blizzaks and it goes just fine thank you. The ESC or whatever works as advertised and helps keep you going when the white stuff flies. The heater is great, the defrosters work great, the ABS is great. So #1, I don't have any issues with it as a winter commuter.
As for daily use, I would swap on Koni yellows and run them at about 1/2 way front and rear. This smooths out the ride a bit, and you can always go full soft up front if things are bumpier where you are. I am in PA and our roads look like the roads in Fallujah, and I honestly don't have any issues. At stock ride height and with 50-series tires, and without a bigger front bar, the Fiesta can handle just about anything. OK, the ride is not like an old Buick, but I prefer to think of it as European and sporty rather than overly stiff. With the Konis, it's all-day comfortable. While it might be a little stiffer than say the Kia, it doesnt punish the kidneys. When I step into anything else, it suddenly feels like a wallowing boat and I want the Fiesta back. So #2, its a great commuter.
I needed a back seat for the kid, but I really like the convenience of a little hatch. I can live with this thing every day (I have bigger cars and trailers for Home Depot runs) because I have a back seat and a hatch. For autocrossing, I can carry a set of tires, tools, food, clothes, whatever I need. It's inside, out of the weather, and safer than on a trailer. I picked up an oversized rake on the way home from an event on Saturday, you can't do that in a triplet! So #3, its a practical car.
And #4, the most important part, it always puts a smile on my face. For my short daily commute, it is so eager and so fun. Every corner exit is a power-on romp up the tach. I trail brake into exit ramps and carry speed through sweepers. It just eggs you on to enjoy the drive. In traffic, it is small, nimble, and able to squirt into gaps. And where it really shines is in the mid-range, where you need to drop a gear and pass a left-lane camper or pull in front of that beige Camry before the twisty two-lane begins. On long drives, you can put it in 6th and save gas, knowing that you can quickly drop a gear or two to get through some traffic. And on the highway with the cruise set at 70, the car is never darty or unstable. It would be a great autobahn car, stable at high speed. Having all that torque is addicting. When I hear about the triplets having dead zones and nothing down low, I know they would not work for me.
So I say go for the Fiesta ST, you won't be disappointed.